Simplify the following expression: $r = \dfrac{-a}{6} + \dfrac{3a}{6}$
Since the expressions have the same denominator we simply combine the numerators: $r = \dfrac{-a + 3a}{6}$ Combine like terms: $r = \dfrac{2a}{6}$ Simplify by dividing numerator and denominator by 2: $r = \dfrac{a}{3}$